BASIC SIBERIAN GENETICS (article 2) By: Todd D.L. Woods, M.D.
Last time you learned the concepts of dominant and recessive genes, autosomal and sex chromosomes, and homozygous and heterozygous genotypes. This time these terms will take on more meaning as the implications of a heterozygous black and white male Br (B=dominant black and r=recessive red) bred to a homozygous red rr (as all recessive traits must be in order to be phenotypically evident) female. Sperm and eggs each carry half of the parents' genetic material. In other words, each of the 39 pairs of chromosomes that were described in the last article, splits into 39 single chromosomes, each half going into one sperm or egg. The distribution of the chromosomes is both ordered and random. It is ordered in that one of each pair separates, for instance the two 14th chromosomes, so that each egg gets a 14th chromosome, rather than one egg getting two 14th chromosomes and not getting any 15th chromosomes. It is random in that which of the 14th chromosomes the egg receives (the one that that parent inherited from its sire or dam) is by chance. In this way the chromosomes that will ultimately make it to an egg will come from both grandparents. The egg, for instance, may get the first copy its 14th chromosome from the maternal granddam and the second copy of its 14th chromosome from the maternal grandsire. The same random and ordered process occurs in the production of sperm. When a sperm and an egg join, each of which consist of a combination of single chromosomes, 1 through 39, from each respective grandsire or dam, the result is 39 pairs of chromosomes. This makes up a complete set of 78 chromosomes which can then become a new, genetically unique puppy. If we trace the heterozygote black (Br) dog and homozygote red (rr) bitch breeding proposed in the first paragraph above we can see how the production and combination of sperm and eggs results in both red and black offspring. (It may also produce grey puppies because another gene can cause the B to be distributed in such a way as to make the overall coat color appear grey--there is not a grey gene per se). Half of the dog's sperm will carry the B gene and half will carry the r gene. All of the bitch's eggs will carry the r gene since this is the only coat color gene she carries. Custom and common sense allows us to use a table to predict the product of the combination of these two individuals: Dog's genes B r Bitch's genes r Br rr r Br rr The dog's sperm will be 50% B and 50% r; the bitch's eggs will be 100% r. Setting these up in the table above we can see how, when the dog's sperm unite randomly with the bitch's eggs, as occurs in fertilization, half of the produce will be Br (black and white or greys) and half will be rr (copper or more dilute). Of course if we do not know the genotype of the dog (whether he is BB or Br) then we could get a different result if a homozygous (BB) black and white dog (who may look the same- same phenotype-as the firstly proposed sire) is bred to the same red bitch. This time we would get. Dog's genes B B Bitch's genes r Br Br r Br Br All the progeny will be black and white or greys though all the progeny will carry the red factor. It is easy to calculate now the predicted percentages of certain traits, assuming they are simple genetics like coat color, if you know the genotype of the proposed parents. Simple genetics is in contradistinction to polygenic and variable penetrance genetics as we will discuss later. As we have discussed coat color genetics in Siberians, there being only two genes, the dominant B for black and the recessive r for red, let's proceed to coat distribution factors. Remember that there is no gene for grey. Another gene that controls for pigment distribution on the individual hairs, can make a black dog grey, or a copper dog light red. This grey or lightening of red factor is usually dominant to the individual hair distribution factor that permits a black (Br or BB) Siberian to appear black or a red (rr) Siberian to appear copper. There are four coat color distribution (coat distribution is different from pigment distribution which determines grey from black) factors in Siberians. The Irish factor, far and away the most common pattern in Siberians, symbolized as si, is a color distribution factor that codes for white on underparts, typical masking on faces, without white located on the upper portion of the dog (i.e. neck or rump) and is dominant to Neck Marks, symbolized as sn. The sn or Neck marks factor allows white on the neck as a small dot up to a full shawl. The sn or Neck marks factor is dominant to Piebald (symbolized as sp) and White (symbolized as sw), but is recessive to Irish. The Piebald factor (sn) is dominant to White (sw). The White factor (sw) is recessive to the other three factors (si, sp, and sn). In other words, in order of decreasing dominance there are Irish (si), Neck marks (sn), Piebald (sp), and White (sw) distribution factors. To test your understanding of dominant and recessive traits answer the following questions: Can a Piebald carry the Irish factor? No! If it did it would be an Irish since Irish is dominant to Piebald. Can a Piebald bitch produce an Irish distributed offspring? Yes! If bred to an Irish factored dog, at least some of that dog's sperm will carry the Irish factor and when combined with the Piebald's coat distribution factor which could be homozygous piebald (sp,sp) or heterozygous piebald (sp,sw), the dog's Irish factor will be dominant and the coat distribution of those puppies will be Irish. Of course if the Irish distributed sire is heterozygous Irish (e.g. si,sn) then we would predict that half of the puppies will be Irish and half will be Neck Marks by the following table. Irish Dog's genes si (Irish) sn (neck marks) Piebald Bitch's genes sp (piebald) si,sp sn,sp sw (white) si,sw sn,sw This table demonstrates that 25% will be si,sp (which would be phenotypically Irish but carry the Piebald gene), 25% will be sn,sp (which would be phenotypically Neck Marks but carry the Piebald gene), 25% would be si,sw (phenotypically Irish but carry the White gene) and 25% would be sn,sw (phenotypically Neck Marks but carry the White gene). Therefore 50% would be phenotypically Irish (the si,sp and si,sw) and 50% would be phenotypically Neck Marks (the sn,sp and sn,sw). Is a white Siberian more or less likely to produce Piebalds? It is more likely to produce them if bred to a piebald, because its distribution factors (sw,sw) are recessive to the piebald's distribution factors (the piebald's being either sp,sp or sp,sw). A white siberian is less likely to produce piebalds than some Neck Mark or Irish Siberians because Neck Mark and Irish Siberians can carry the sp factor, hidden behind the dominant Neck Mark (sn) or Irish (si) factors. Please take note that when I refer to white, I am referring to a real white, with either black or liver points, and not to an albino or to an Isabella white. An Isabella white, is a creamy white that is caused by an "oxidizing" of color such that a black, grey or red Siberian becomes a creamy yellowish casted white--the so called Isabella white. This type of white is not determined by a distribution factor, it is determined by a melanin factor. The melanin factor is symbolized by C for dominant (the common Siberian gene) and cr for the "oxidizing" Isabella white gene. An Isabella white is cr,cr. An rr Isabella white has liver points (nose, eye rims and lips) and is a red Siberian in disguise. A Br or BB Isabella white has black points and is a grey or black and white Siberian in disguise. Before we leave simple dominant and recessive traits let me explain what variable penetrance means. Penetrance is a term to describe the extent to which a dominant allele (or gene) will reveal itself when present in only a single dose (e.g. si,sp). If this combination produced 100% Irish (as it does) then the Irish factor would be said to have 100% penetrance. If this combination produced Irish distributed coat color in 75% of individuals, the Irish gene would be said to have 75% penetrance. Simple dominant or Mendelian dominant traits are ones with 100% penetrance. Some genes are dominant with variable penetrance (e.g. probably some forms of epilepsy and temperaments) which then produces different implications for breeders dealing with these problems, because apparently unaffected individuals may nevertheless carry the undesirable genes. Coat color and distribution genetics can be considered to have simple or "Mendelian dominance". You will remember that in the first article, I stated that (for Gregor Mendel and the field of genetics) "Serendipitously, pea genetics are very straight forward." Pea genetics are simple dominants and recessives. This fact allowed insight to theorize and then prove the genetics of pea plant height. In honor of Gregor Mendel, the term Mendelian genetics is used to describe genetics that behave according to simple dominant and recessive models. If you become fluent in the terms and genetic factors in this and the first article, you will have a strong handle on the determination of coat color and distribution in Siberians and will have the beginning of the tools necessary to approach upcoming subjects like hip dysplasia, temperaments, cataracts, PRA (progressive retinal atrophy), glaucoma, etc. Lessons in this column: B Black dominant coat color symbol r Red recessive coat color symbol si Irish coat color distribution factor symbol sn Neck Marks coat color distribution factor symbol sp Piebald coat color distribution factor symbol sw White coat color distribution factor symbol C Non "oxidizing" melanin factor cr "oxidizing" melanin factor that in homozygous form produces Isabella white penetrance: the extent to which a dominant factor reveals itself when present in only a single dose polygenic: relates to a trait that is controlled by several genes each of which have a small effect but which have a considerable effect when present together. Mendelian or simple dominance: dominance with 100% penetrance
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